If you get a steak that is just called a sirloin steak, it is usually from the top sirloin, as the bottom is made into larger, tougher cuts that are better for roasting.
Essentially, ribeyes are cut from the rib roast as individual steaks before they are cooked. They can be cut bone-in or cut boneless from the space between the rib bones.
Ribeyes steaks are special for a lot of reasons, but it's most famous for its excellent fat marbling. It is perfectly flecked and streaked through the steak.
Ribeye is much more specific than sirloin, referring to that one small area of the rib. There are a few different styles of ribeye beyond bone-in or boneless.